As the content of the World Wide Web (or “web”) grows richer, more users rely on search engines to locate relevant information, thereby making search engines one of the most-used applications of modern information technology. Large web search engines may process billions of queries each day and return results from over tens of billions of documents.
In general, the stringent requirements for a user's positive search experience are low latency and highly relevant search results. However, the load on search engines can vary drastically when, for example, search engine servers become unavailable due to popular events happening around the world, or because of coincidental spikes in global user activity. Often when the query load exceeds the search engine's capacity, excess queries get dropped, and from the user's perspective this can be a very poor experience, perceived as an ungraceful degradation in search quality.